1. Field of the Invention
The present invention relates to a method of producing a single crystal and an apparatus used therefor.
2. Description of Related Art
There are various methods of growing a single crystal, one of which is a pulling method called Czochralski method (CZ method).
FIG. 1 is a longitudinal sectional view schematically showing a conventional apparatus for producing a single crystal used For the CZ method. In FIG. 1, numeral 3 designates a crucible arranged in a chamber. The crucible 3 includes a bottomed cylindrical quartz inner-layer container 3a and a bottomed cylindrical graphite outer-layer container 3b fitted on the outside of the inner-layer container 8a. A heater 4 of resistance heating type is arranged concentrically with the crucible 3 on the outside of the crucible 3. The crucible 3 is Filled with a molten liquid material 7 melted by the heater 4. A lift axis 12 of a lift rod, wire or the like is suspended above the center of the crucible 3. A seed crystal 18 is set at the lower end of the lift axis 12. In growing a single crystal 8, the lift axis 12 is pulled up after the seed crystal 13 is made in contact with the surface of the molten liquid material 7. Then, the molten liquid material 7 attached to the seed crystal 13 solidifies and grows to form a single crystal 8.
According to the CZ method, impurity is often added to the molten liquid material 7 before the pulling process in order to adjust the electrical resistivity and electrical conduction type of the single crystal 8. The problem in this case is the segregation of impurity which may occur along the longitudinal direction of the single crystal 8. This segregation is caused by the Fact that the ratio C.sub.s /C.sub.L in the boundary between the molten liquid material 7 and the single crystal 8, i.e., the effective segregation coefficient K.sub.e, is not 1, where C.sub.s is the impurity concentration of the single crystal 8 and C.sub.L the impurity concentration in the molten liquid material 7. Generally, K.sub.e is often less than 1, in which case the impurity concentration in the molten liquid material 7 gradually rises with the growth of the single crystal 8. The natural result of this is that a single crystal 8 having electrical characteristics lacking uniformity along the longitudinal direction is produced.
The Double Layered CZ method is known as a method of suppressing the segregation. FIG. 2 is a longitudinal sectional view schematically showing an apparatus for producing a single crystal used for implementing the Double Layered CZ method. In the conventional apparatus shown in FIG. 2, the heater 4 is arranged at a position higher than that in the apparatus shown in FIG. 1. According to the Double Layered CZ method, a solid layer 21 of a crystal material is arranged in the lower part of the crucible 3 and a molten liquid layer 22 of the crystal material is formed above the solid layer 21 by subjecting the heater 4 to control in such a manner as to allow the solid layer 21 and the molten liquid layer 22 to coexist. After making the seed crystal 13 in contact with the molten liquid layer 22, a lift axis 12 is pulled up to grow a single crystal 8. According to this method, the solid layer 21 is melted as the single crystal 8 grows, so that the impurity concentration in the molten liquid layer 22 is kept constant to suppress the segregation. This method has another advantage in that it can be implemented in the apparatus used for the CZ method by controlling the partial heating of the heater.
In the aforementioned Double Layered CZ method, the central portion of the molten liquid layer 22 in close proximity to the solid layer 21 often solidifies into undesirable contact with the single crystal 8 being pulled up during crystal growth. This condition is shown in FIG. 3 and is very dangerous. The contact, once happened, not only damages the single crystal 8 itself but also, in case of shaft type, breaks the seed crystal and generates the Fall down of the crystal, or in case of wire type, leads to a secondary damage such as a twisted wire and a broken wire.